1. Field of the Invention
This invention pertains to bench blasting methods and, in particular, to a method of selecting the placement of boreholes along a drill line.
In modern bench blasting, vertical or near vertical holes are drilled adjacent to a rock face and are loaded with explosive charges that are then detonated. The detonation fractures the rock mass between the borehole and the rock face and displaces the resulting fractured rock. The resulting broken rock, known as “muck”, is removed and a new free rock face is thus exposed. If the muck contains a desired product, it can be gathered and processed. Otherwise, it may simply be removed from the blasting site to permit further blasting or other activities.
2. Related Art
U.S. Pat. No. 3,377,909 to Grant et al, issued Apr. 16, 1968, discloses the use of a “powder factor” (cubic yards of earth per pound of explosive) to characterize a borehole pattern in a coal field strip mine and discloses a “normal” spacing for ANFO (see col. 7, lines 70-71 and 6, lines 63-68).
U.S. Pat. No. 3,848,927 to Livingston, dated Nov. 19, 1974, discloses a trial and error method of determining the optimum and critical depths of a small charge, and teaches the scaling-up of this information for larger charges based on cube root scaling (see col. 7, lines 645). This patent suggests matching the charge to the desired size of debris.
U.S. Pat. No. 4,273,049 to Edwards et al, dated Jun. 16, 1981, suggests overcoming the dampening effect of water in a borehole by using water-resistant explosive in the water-containing portions of the borehole and conventional explosives above those portions.
U.S. Pat. No. 4,440,447 to Ricketts et al, dated Apr. 3, 1984, teaches that, in a borehole array for the formation of a retort in oil shale, outer boreholes can be closely spaced and made smaller in diameter to maintain the powder factor (see col. 8, lines 40-53), which is defined as the ratio of energy or explosive used per unit volume of formation explosively expanded in pounds ANFO equivalent per ton of oil shale formation expanded (see column 10, lines 13-17). No explanation of the term “ANFO equivalent” is given.
International Patent Application PCT/GB90/00567, which is incorporated herein by reference or background information, discloses a laser rangefinder device referred to by the trademark QUARRYMAN that can be used to survey a rock face and, when given a borehole pattern by the user, to calculate the burden associated with each borehole. This patent application also discloses a borehole analyzer referred to by the trademark BORETRAK that allows the user to determine the configuration of a borehole as actually drilled.
Prior art methods for assessing the rock hole burden associated with a given borehole or, alternatively, for predicting the optimum positions for boreholes along a rock face, made use only of gross approximations of the burdens associated with the boreholes. Typically, the volume of explosive material in the borehole is calculated and a known conversion factor corresponding to the powder factor disclosed in U.S. Pat. No. 3,377,909 (discussed above) is used to project a volume of rock to be associated with the explosive material in the borehole, i.e., the rock burden. The rock burden is then expressed as a roughly rectangular block, one dimension of which corresponds to the length of the column of explosive material in the borehole, another to the distance of the borehole to the rock face. The projected hole spacing along the drill line can then be derived as the third dimension of the rectangular block. This calculation method is highly inefficient because it does not take into account significant variations in the configuration of the rock face that can occur within the dimensions of the rectangular block associated with the borehole.